The invention relates to a wheel slip control system (ASR) for motor vehicles wherein, when a driven vehicle wheel spins it is braked, and when both driven wheels spin, the engine power is also reduced. A spinning of the driving wheels is sensed by rotational speed sensors and fed to an electronic control unit which creates a slip signal when a differential speed threshold value between the driven and the non-driven vehicle axle is exceeded. The electronic control unit controls engine power downward during the duration of the slip signal through a control element. When the spin signal drops out, the engine power is controlled upward to the desired gas pedal value set by the driver. The period for control is set by a time function element in the electronic control unit.
A wheel slip control system of this general type is known (DE-OS 35 44 294) wherein the engine power at the start and at the end of the wheel slip control operation is controlled such that, during the downward control as well as during the upward control, the power follows first a steep and then a flat rise. In this case, a time function element is provided which can be controlled by an electronic control unit and contributes to achieving the desired steep-curve characteristics.
Wheel slip control systems have the purpose of preventing an uncontrolled spinning of the driving wheels when a vehicle is started on a smooth ground surface. When spinning occurs, the starting time of the vehicle is prolonged unnecessarily, its tractive power is reduced and cornering force is diminished.
The spinning of the wheels is sensed by rotational speed sensors arranged at the wheels and is analyzed in an electronic control unit. If only one wheel spins, the pertaining wheel brake is actuated by means of a valve control of a brake. As a result, by means of the wheel differential, a drive torque is transmitted to the other drive wheel.
When both driving wheels spin, however, the power of the driving engine is controlled downward. For this purpose, intervention with the throttle linkage causes the value set by the driver to be reduced.
The wheel slip engine control system therefore normally becomes operative when a certain speed difference exists between the driven and the non-driven axle. When one falls below this speed difference threshold, the engine control switches off again with the power of the engine controlled upward to the value set by the driver by means of the gas pedal.
When the vehicle is loaded and the tractive-force level is high (e.g. rough ice, snow), a brief spinning of the driving wheels frequently occurs and triggers the wheel slip engine control. As soon as the power is reduced a little, adhesion with respect to the road surface is reestablished and the engine power is increased again, which may result in a renewed spinning of the driving wheels. In the worst case, when the cycle is continually repeated, strong drive train vibrations may occur which require special interventions by the driver.
It is the object of the invention to develop a wheel slip control system of the above-mentioned type in such a manner that drive train vibrations can no longer occur.
According to the invention, this object is achieved if an output signal is generated when spin signals occur at intervals below a certain limit time. This actuates the time function element to prolong the actuation of the downward-controlling control element beyond the duration of the last spin signal.
It is advantageous if the electronic control unit again generates an output signal, when a given time period has not been passed between another spin signal and the dropping of the preceding spin signal.
It is also advantageous if the time function element automatically switches to a longer deceleration time period after the expiration of the first deceleration time period, but automatically switches back to the first declaration time period if it is not activated again by the output spin signal within a given time. The time function element, after the expiration of the longer deceleration time period, automatically switches over to a still longer deceleration time period; after the expiration of that still longer deceleration time period, to an even longer deceleration time period, then the still long longer time period, etc. However, the time function element will automatically switch back to the first extension time period if, within a given time period, it is not again activated by an output spin signal.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.